Evolution of Passivity for the Multi-Principal Element Alloy CoCrFeNi with Potential, pH and Exposure in Chloride Solution

The evolution of passivity of the multi-principal element alloy CoCrFeNi was studied as a function of potential, pH and exposure duration in 0.1 M NaCl. It was shown that CoCrFeNi exhibits excellent passivity irrespective of pH, revealing a multi-oxide passive film enriched with Cr(III) oxide. Electrochemical impedance spectroscopy suggests that the passive film thickness and polarisation resistance increase with pH and exposure duration, whereby the growth behavior of the passive film was consistent with the assumptions of the point defect model. X-ray photoelectron spectroscopy analysis suggested that the fraction of Co(II) and Ni(II) oxides in the passive film, and their contributions to the passivity of the alloy, increased with increase in pH of the electrolyte. The present work explores the complex synergy between composition, thermodynamics and kinetics on the resultant passivity of a multi-principal element alloy.

STUDY OF THE PASSIVITY LIMITS OF AUSTENITIC STAINLESS-STEEL IN H2S-CONTAINING BRINES USING MOTT-SCHOTTKY AND POINT DEFECT MODEL ANALYSIS

The objective of the study is to correlate the effects of H<sub>2</sub>S, and Cl<sup>-</sup> concentration on the passivity limits and the onset of localized corrosion, in UNS S31603 stainless steel by evaluating the changes in the semiconducting behavior exhibited by the passive layer. The study is accomplished experimentally by using a combination of direct and alternate current electrochemical methods, to study the passive layer formed by the stainless steel, in equilibrium with a gas phase at 2.8 MPa (400 psi) containing up to 60% mol of H<sub>2</sub>S (bal. CO<sub>2</sub>) at 25○</sup>C. The results obtained using the Mott-Schottky analysis indicate that the decrease of the passive layer stability formed on the UNS S31603 stainless steel is consistent with the increase in the electron donor carrier density. This is observed as the consequence of the effect of Cl<sup>-</sup> and H<sub>2</sub>S. In this context, the Cl<sup>-</sup> content in the brine was found to exert a larger effect than the H<sub>2</sub>S activity. The correlation with the evaluation of the passive layer using the Point Defect model suggest that both the polarizability and the rate of annihilation of the cation vacancies at the metal / film interface increase with the H<sub>2</sub>S content in the environment. This behavior can explain the increased content of Cl<sup>-</sup> and sulfides as main electron donor species, also the observable increase in the passive layer susceptibility to both stable and metastable pitting.

Study on the aerosol generation of plutonium metal due to oxidation

Transuranic metals such as plutonium usually need to be protected in inert gases to inhibit oxidation and produce aerosols. In the paper, the oxidation reaction process of plutonium was depicted based on the point defect model. The aerosol generation rate was approximated to the oxidation rate of plutonium when the oxidation layer keeps constant. The results show that the rate is strongly dependent on the vacancy diffusivity and reaction rate constant of the vacancy generation and consumption. The aerosol generation rate at the temperature of 298 K is in the order of 108 particles/cm2/s.